1. Field of the Invention
The invention relates to a core substrate and a method for fabricating a circuit board. More particularly, the invention relates to a core substrate and a method for fabricating a circuit board that applies the core substrate.
2. Description of Related Art
Generally speaking, when desiring to form a circuit board having an embedded circuit, a core substrate is provided first. This core substrate consists of a dielectric layer, a first copper foil layer and a second copper foil layer that are stacked in sequence, wherein the first copper foil layer directly covers the dielectric layer, and the second copper foil layer directly covers the first copper foil layer. Herein, a thickness of the first copper foil layer is 18 μm, and a thickness of the second copper foil layer is 5 μm. The second copper foil layer may be viewed as an ultra-thin copper foil and has a higher production cost. Next, a patterned circuit layer is formed on the second copper foil layer. Next, an insulating layer is formed on the patterned circuit layer, so as to cover the patterned circuit layer and the second copper foil layer exposed outside the patterned circuit layer. Then, the first copper foil layer and the second copper foil layer are separated from each other by means of a special fixture. Finally, the second copper foil layer is removed by an etching step, so as to expose the insulating layer and the patterned circuit layer, thereby completing fabrication of a conventional circuit board having an embedded circuit.
However, as the cost of the second copper foil layer that is viewed as an ultra-thin copper foil is higher, the production cost of the entire circuit board is increased. Moreover, since a special fixture is required for separating the first copper foil layer and the second copper foil layer from each other, this step further increases the production cost of the entire circuit board by requiring this special fixture. In addition, when removing the second copper foil layer by the etching step, since an etching liquid etches the patterned circuit layer at the same time, surface flatness of the patterned circuit layer is reduced so as to affect structural reliability of the circuit board.